The invention relates to an apparatus and method for preventing instabilities in the handling of a vehicle, and, more particularly, to such a method in which a desired value of the vehicle yaw angle rate .mu..sub.des is formed in a computer unit from measured values (namely, vehicle speed, steering wheel angle and the like), in which there is furthermore supplied to the computer unit at least one sensor signal from which the actual value of the vehicle yaw angle rate .mu..sub.act is formed with, the difference between the desired value of the yaw angle rate .mu..sub.des and the actual value of the yaw angle rate .mu..sub.act being formed in the computer unit, and the actual value of the yaw angle rate .mu..sub.act is subtracted from the desired value of the yaw angle rate .mu..sub.des, and at least one output signal, is generated by the computer unit, from this difference, in which the output signal represents the detected driving situation with respect to the yaw behavior of the vehicle, and the brake slip of individual vehicle wheels is varied as a function of this output signal.
An instability preventing method is disclosed in DE 36 25 392 A1, according to which, the yaw angle rate .mu..sub.act of a vehicle is measured, for example by means of a fiber-optic gyro, in order to detect the driving situation with respect to the yaw behavior of the vehicle. An alternative way of determining the actual value of the yaw angle rate .mu..sub.act is by deriving the yaw angle rate .mu..sub.act by using at least one acceleration sensor which measures the radial acceleration of the vehicle. Furthermore, a desired value for the yaw angle rate .mu..sub.des can be derived from the measured vehicle speed in the longitudinal direction and the measured steering angle. In the latter, a critical driving situation is derived when the actual value of the yaw angle rate .mu..sub.act deviates from the desired value of the yaw angle rate .mu..sub.act, i.e. when the actual behavior of the vehicle deviates from the desired behavior of the vehicle. This detected deviation of the actual behavior from the desired behavior of the vehicle is then used in order to minimize the deviation of the actual behavior of the vehicle from the desired behavior of the vehicle, in that an automatic intervention in the steering takes place and/or in that individual wheels of the vehicle are braked or accelerated such that the deviation is minimized.
A so-called linear single-track model of a vehicle is disclosed in other literature references (DE Book: Zomotor, Adam; Fahrwerktechnik [Running Gear Technology]: Fahrverhalten [Handling]; Publisher: Jornsen Reimpell; Wurzburg: Vogel, 1987; 1st edition; ISBN 3-8023-0774-7, in particular pages 99-127), by way of which a yaw angle rate .mu..sub.act of the vehicle, which under specific conditions is self-adjusting and is then used, on the basis of this single-track model, as the desired value of the yaw angle rate .mu..sub.des can be determined, for example, from measured values of the vehicle speed in the vehicle longitudinal direction and the steering wheel angle or the steering angles of the wheels corresponding thereto.
An object of the present invention is to provide a method and apparatus for preventing instabilities in vehicle handling as early as possible.
This object has been achieved according to the present invention by a method in which the time derivative of the difference between the actual and desired yaw angle rate is formed in the computer unit, with the output signal being generated in the computer unit as a function of this time derivative. The output signal contains information on whether the vehicle has oversteering or understeering handling. In the case of oversteering handling, the brake slip is increased for the vehicle front wheel on the outside of the turn, and in the case of understeering handling, the brake slip is increased for the vehicle rear wheel on the inside of the turn.
One of the advantages of the invention is that, as a result of the early detection of the driving situation with respect to the yaw behavior of the vehicle, unstable driving situations can be detected very early. It is thus possible to prevent the possible occurrence of unstable driving situations very early by a variation or increase in the brake pressure to change the slip .sigma. on individual wheels.
The vehicle longitudinal speed and the steering wheel angle or steering angle of the wheels are detected by suitable sensors. These sensor signals can be supplied to a computer unit in which a yaw angle rate .mu..sub.des of the vehicle desired by the vehicle driver can be determined from these variables (for example in accordance with the linear single-track model) as the desired value of the yaw angle rate .mu..sub.des. The driving situation with respect to the yaw behavior is then detected in the computer unit, in that the actual value of the yaw angle rate .mu..sub.act is compared with the determined desired value .mu..sub.des. In this situation, not only the magnitude of the difference between the actual value of the yaw angle rate .mu..sub.act and the desired value .mu..sub.des is considered, but also the mathematical sign of this difference and the time derivative of this difference. Particularly early detection of the possible occurrence of critical driving situations is possible, especially as a result of the consideration of the time derivative, so that the occurrence of critical driving situations can then be prevented by a suitable variation or increase in the brake pressure to vary the slip .sigma. on individual wheels.
In the case of oversteering handling in which the vehicle pulls itself into the turn, the front wheel on the outside of the turn is braked. The decrease in the lateral guiding force and the increase in the braking force in the circumferential direction result in a yaw moment of the vehicle which turns it back. The oversteering behavior of the vehicle is thereby decreased. If necessary, the stabilizing effect is reinforced by also additionally braking the wheel on the inside of the turn. This reinforcement occurs since, although the braking force supports the turning-in process in the circumferential direction, the decrease in the lateral guiding force of the wheel on the inside of the turn, however, not only compensates for the effect but, because of the lever arm of the forces acting, overcompensates and thus additionally stabilizes the vehicle.
In the case of understeering handling in which the vehicle is unwilling to yaw, i.e. it does not follow the steering input prescribed by the vehicle driver, the rear wheel on the inside of the turn is braked. The decrease in the lateral guiding force of this wheel and the increase in the braking force in the circumferential direction result in a yaw moment turning inwards. If required, the rear wheel on the outside of the turn can additionally be braked in this case. In a manner analogous to the abovementioned criterion for braking the front wheel on the inside of the turn in the case of oversteering handling, it is expedient if the product of the lateral force loss and the associated lever arm to the center of gravity is greater than the product of the braking force in the circumferential direction and the associated lever arm to the center of gravity.
As the actual value .mu..sub.act of the yaw rate approaches the desired value .mu..sub.des, the braking forces are correspondingly decreased.
As an alternative to determining the desired value of the yaw angle rate .mu..sub.des by way of the linear single-track model, it is also possible to read this desired value of the yaw angle rate .mu..sub.des from a performance graph which is measured once.